Method for producing La/Ce/MM/Y base alloys, resulting alloys and battery electrodes

ABSTRACT

A carbothermic reduction method is provided for reducing a La-, Ce-, MM-, and/or Y-containing oxide in the presence of carbon and a source of a reactant element comprising Si, Ge, Sn, Pb, As, Sb, Bi, and/or P to form an intermediate alloy material including a majority of La, Ce, MM, and/or Y and a minor amount of the reactant element. The intermediate material is useful as a master alloy for in making negative electrode materials for a metal hydride battery, as hydrogen storage alloys, as master alloy additive for addition to a melt of commercial Mg and Al alloys, steels, cast irons, and superalloys; or in reducing Sm 2 O 3  to Sm metal for use in Sm—Co permanent magnets.

RELATED APPLICATION

This application is a continuation of international PCT application No.PCT/US2011/001277 filed Jul. 19, 2011, which claims benefits andpriority of U.S. provisional application Ser. No. 61/399,943 filed Jul.20, 2010, the entire disclosures of which are incorporated herein byreference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Grant No.DE-AC02-07CH11358 awarded by the Department of Energy. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates rare earth-based alloys comprising amajority of a rare earth selected from lanthanum (La), cerium (Ce),mischmetal (MM), and/or yttrium (Y) and to a method of making the alloysby carbothermic reduction of a La-, Ce-, MM- and/or Y-containing oxide,and to battery electrodes comprised of such rare earth-based alloys.

BACKGROUND OF THE INVENTION

Researchers over the last 100 years have made various attempts toprocess pure refractory and rare earth metals and alloys by thecarbothermic process. The driving force for all of these investigationshas been the low cost of carbon as the reducing agent (reductant) andthe simplicity of processing with no slag formation. Some success wasachieved by these workers, but high purity materials were not obtainedand consistently contained large amounts of carbon (usually as metalliccarbides), unreduced oxides, and other interstitials.

In the manufacture of nickel metal hydride batteries, a commonly usedalloy for the negative electrode [where hydrogen ions (protons) arestored as metal hydride] comprises La(Ni,Co)_(4.5)Si_(0.5). The negativeelectrode alloy can be prepared by reacting a La-based alloy LaSi_(0.5)with nickel (Ni) and cobalt (Co) in direct proportions which are usuallyclose to equal amounts of Ni and Co.

Mischmetal (MM) comprises a majority of La, Ce collectively and minoramounts of other rare earth elements depending on the source of the ore.MM also can be used in the nickel metal hydride battery electrode suchthat the above-described reaction using a MM-based alloy also appliesfor preparing MM(Ni_(1-x)Co_(x))Si_(0.5) electrode materials.

A discussion of materials for metal-hydrogen batteries is provided by T.Sakai, M. Matsuoka and C. Iwakura, “Rare Earth Intermetallics forMetal-Hydrogen Batteries,” in Handbook on the Physics and Chemistry ofRare Earths, Ed. K. A. Gschneidner, Jr. and L. Eyring, Vol. 21, Chap.142, pp. 133-178 (1995).

There is a need to prepare La based alloys from La₂O₃ in a manner thatis less expensive than current processes, which use calcium (Ca) as areducing agent or which involve electrolytic reduction of LaF₃ or LaCl₃to La metal with high consumption of electricity.

SUMMARY OF THE INVENTION

The present invention provides in an embodiment, a carbothermicreduction method wherein a La- and/or Ce-containing oxide is/arecarbothermically reduced in the presence of carbon and a source of areactant element X selected from the group consisting of silicon (Si),germanium (Ge), tin (Sn), lead (Pb), arsenic (As), antimony (Sb),bismuth (Bi), and phosphorous (P) to form an intermediate alloy materialcomprised of a majority of La and/or Ce and a minor amount of X whereinX is selected from the group consisting of silicon (Si), germanium (Ge),tin (Sn), lead (Pb), arsenic (As), antimony (Sb), bismuth (Bi), andphosphorous (P). Other oxides such as MM-containing oxide and/orY-containing oxide can be similarly carbothermically reduced pursuant toother embodiments of the invention to produce MM-based alloys or Y-basedalloys. The rare earth-based alloy material of a particular embodimentof the invention can include X, such as Si, in minor amount from about 5to about 50 atomic %, such as commonly about 5 to about 40 atomic % X,with the majority of the alloy (greater than 50 atomic %) being La, Ce,MM, and/or Y. The source of the reactant element can comprise elementalSi, Ge, Sn, Pb, As, Sb, Bi, and/or P, the oxides thereof, and/or othercompounds thereof, that can participate in the carbothermic reductionreaction to form the intermediate material.

This intermediate rare earth-based material is useful as a master alloyin making a negative electrode material (as a hydrogen storagecomponent) for a metal hydride battery. For example, the presentinvention provides in another embodiment, a method wherein thecarbothermically reduced La-, Ce-, or MM-based intermediate alloymaterial including Si (as a master alloy) is alloyed with a transitionmetal such as Ni to make an electrode material for a metal hydridebattery. The present invention in embodiments include a negativeelectrode comprising a carbothermically reduced rare earth elementincluding La, Ce, and/or MM, an element selected from the groupconsisting of Si, Ge, Sn, Pb, As, Sb, Bi, and P that participated in thecarbothemic reduction of the rare earth element, and a transition metalsuch as Ni, which may be partially substituted by one or more otherelements such as including, but not limited to, Co.

The intermediate rare earth-based material also is useful as a masteralloy in reducing Sm₂O₃ to samarium metal (Sm) for use in Sm—Copermanent magnets, or other Sm-metal containing products.

The intermediate rare earth-based material also is useful as a masteralloy additive to a melt of commercial Mg and Al alloys, steels, castirons, and superalloys to introduce the rare earth element into themelt.

Advantages of the present invention will become more readily apparentfrom the following detailed description taken in conjunction with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the oxidation of La₅Si_(2.5) at ambient roomtemperature.

FIG. 2 is a pressure-composition of the hydrogenation [hydrogen (H)content as a function of hydrogen pressure] of an electrode material,La(Ni, Co, Si)₅ where Y may be B, Al, Si, Ti, V, Cr, Mn, Fe, Cu, Zn, Mo,or combinations thereof made pursuant to an embodiment of the invention.

FIG. 3 is a plot of voltage versus time showing the charging behavior ata current of 20 mA over a 14 hour period of an electrode pursuant to anillustrative embodiment of the invention.

FIG. 4 is a plot of voltage versus time showing the dischargingcharacteristics for a discharging current of 10.6 mA for the electrodeof FIG. 2.

FIG. 5 is a plot of specific capacity versus cycles showing the capacityof a test cell as a function of the number of charging/dischargingcycles.

FIG. 6 is a plot of the oxidation of Ce₅Si_(2.6) at ambient roomtemperature.

FIG. 7 is a plot of the oxidation of MM₅Si₃ at ambient room temperature.

FIG. 8 is a plot of the oxidation of Y₅Si_(2.6) at ambient roomtemperature.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides in an embodiment a carbothermic reductionmethod for reducing a La- and/or Ce-containing oxide at temperaturesbelow 1800 degrees C. and above about 1300 degrees C. The La- and/orCe-containing oxide is reduced in the presence of carbon (reducingagent) and a source of a reactant element X described below in order toform a La- and/or Ce-based intermediate alloy material. While notwishing to be bound by any theory, it is believed that the reactantelement catalyzes, and/or acts as a flux, and/or otherwise participateschemically in the carbothermic reduction of the rare earth oxide. TheLa- and/or Ce-containing oxide can include La₂O₃, CeO₂, mixtures ofLa₂O₃ and CeO₂, and mischmetal (MM) oxides that have a majorityconcentration of La₂O₃ and CeO₂ collectively and other rare earthoxides, such as Pr₆O₁₁ and Nd₂O₃, depending on the source of the ore.Other rare earth elements such as Pr and/or Nd as well as non-rare earthelements such as Zr could be substituted for a portion of the rare earthconstituent, and thus their oxides would be co-reduced with La₂O₃ and/orCeO₂.

The carbothermic reduction process is a solid state, diffusioncontrolled process and intimate contact between the carbon reducingagent and the oxide particles and source of a reactant element Xselected from the group consisting of Si, Ge, Sn, Pb, As, Sb, Bi, and Pis employed for the reduction to reach completion. The optimum particlesize of the La and/or Ce-containing oxide, carbon, and source of the Si,Ge, Sn, Pb, As, Sb, Bi, and/or P reactant element and the bestconditions for milling and blending the mixture thereof can bedetermined empirically to this end. The Examples below illustratecertain exemplary parameters for carrying out the carbothermic reductionreaction.

Although Example 1 described below starts with La₂O₃, the invention canbe practiced using oxides of La, Ce, MM (which can include Nd and/or Pr)and/or Y (see Examples 2, 3 and 4, respectively) using similar reactiontemperatures and times as set forth below. The fine La₂O₃ powder ismixed and blended with carbon and Si, (both are also fine powders),formed into a paste by adding acetone containing 3 wt. % polypropylenecarbonate as a binder, and formed into briquettes which are air dried at100° C. for a few hours. The briquettes are heated in a vacuum furnacefor example capable of a vacuum of 5×10⁻⁵ Torr or less in various stepsuntil about 1700° C. when the entire mixture is melted for a time toreact C and O to reduce their content in the alloy. The by-products areCO and a small amount of SiO gas. The resultant ingot when cooled is aLaSi_(x) alloy and may contain about 5 atomic % to as much as about 50atomic % Si (about 18 wt. % Si), such as commonly about 5 atomic % toabout 40 atomic % Si. The one-step method of the invention is muchcheaper than current processes for making La metal because carbon is amuch less expensive reducing agent than Ca; and in the case of anelectrolytic method, the cost of electricity to reduce LaF₃ and/or LaCl₃to La metal is quite high. Furthermore, it is a much moreenvironmentally friendly technology because in the other reductionmethods CaF₂ or CaCl₂ slag is produced and must safely be returned tothe environment; while in the electrolysis method the byproduct is thehazardous fluorine (F₂) gas. In the carbothermic-silicide method of theinvention, the CO by-product can be used as a starting material forpreparing organic compounds or as a component of producer gas (alsoknown as water gas) for the cogeneration of heat or electricity.

It is noted that although this description provides some specificexamples of rare earths, such as La, Ce, and Y, the principles of thepresent invention can be applied to other rare earth elements. Forexample, in some embodiments, the carbothermic reduction method can beused to reduce oxides that include one or more of Sc, Y, La, Ce, Pr, Nd,Gd, Tb, Dy, Ho, Er, and Lu, and consequently, in embodiments, theintermediate alloy generated from the carbothermic reaction (and anyother products generated from the intermediate alloy such as anelectrode of a battery) can include one or more of Sc, Y, La, Ce, Pr,Nd, Gd, Tb, Dy, Ho, Er, and Lu.

A master starting alloy produced by carbothermic-catalytic reductionincludes commonly from about 70 to about 98 wt. %, more commonly fromabout 75 to about 95 wt. %, and even more commonly from about 80 toabout 90 wt. % rare earth; commonly from about 2 to about 35 wt. %, morecommonly from about 5 to about 30 wt. %, and even more commonly fromabout 10 to about 25 wt. % reactant element; and commonly from about0.005 to about 2 wt. %, more commonly from about 0.05 to about 1.5 wt.%, and even more commonly from about 0.5 to about 1 wt. % carbon,oxygen, and nitrogen (individually and collectively).

In the master starting alloy, a molar ratio of rare earth to reactantelement typically ranges from about 1:1 to about 3:1, more typicallyfrom about 1.1:1 to about 2.5:1, and even more typically from about1.2:1 to about 2:1.

While not wishing to be bound by any theory, the chemical reactions ofthe carbothermic-silicide method of illustrative embodiments of theinvention can be described as follows:La₂O₃+(3−x)C+(1+x)Si→2LaSi_(0.5)+(3−x)CO↑+xSiO↑  (1)Ce₂O₂+(2−x)C+(0.5+x)Si→CeSi_(0.5)+(2−x)CO↑+xSiO↑  (2)MM₂O₃+(3−x)C+(1+x)Si→2MMSi_(0.5)+(3−x)CO↑+xSiO↑  (3)Y₂O₃+(3−x)C+(1+x)Si→2YSi_(0.5)+(3−x)CO↑+xSiO↑  (4)

The resulting LaSi_(0.5) of Eq. (1) provides a master starting alloy formaking a variety of products. While not wishing to be bound by anytheory, in making a Ni metal hydride battery, a common alloy used forthe negative electrode (hydrogen absorbing electrode where oxidationoccurs during battery discharge) is La(Ni,Co)_(4.5)Si_(0.5), which canbe prepared by reacting LaSi_(0.5) with the Ni and Co (in the correctproportions, usually close to equal amounts of Ni and Co):LaSi_(0.5)+4.5(Ni_(1-x)Co_(x))→La(Ni_(1-x)Co_(x))_(4.5)Si_(0.5), where xis 0 to 0.75  (5)

Since MM is also used in the Ni metal hydride batteries, this reactionalso applies for making MM(Ni_(1-x)Co_(x))_(4.5)Si_(0.5) electrodematerials where MM is La, Ce and other rare earth elements.

Chemical reactions involving CeO₂ and MM₂O₃ of Eq. (2) and (3) willresult in CeSi_(0.5) and MMSi_(0.5) master start alloys for making avariety of products. These include; CeSi_(0.5) and MMSi_(0.5) alloys asadditives to Mg- or Al-based alloys, steels, cast irons, and ductileirons; and MMSi_(0.5) for the manufacture of lighter flints. Similarlyone can use Y₂O₃ to make YSi_(0.5) of Eq. (4) as a master start alloyfor Y additions to superalloys to inhibit oxidation and corrosion of thesuperalloys, and to Mg-base alloys to improve the high temperature creepresistance of the Mg-base alloys.

The LaSi_(0.5) (or CeSi_(0.5)) alloy also may be used as a reducingagent to prepare Sm metal as the starting material for makingSmCo₅/Sm₂Co₁₇ permanent magnet materials. While not wishing to be boundby any theory, the chemical reaction for preparing the Sm metal fromLaSi_(0.5) is as follows:6LaSi_(0.5)+(3+x)Sm₂O₃→(6+2x)Sm↑+3xSiO↑+3La₂O₃+(3−3x)Si  (6)6CeSi_(0.5)+(3+x)Sm₂O₃→(6+2x)Sm↑+3xSiO↑+3Ce₂O₃+(3−3x)Si  (7)

Free Si will be one of the final products because the free energy offormation at 1300 K of La₂O₃ (or CeSi_(0.5)) is nearly twice as negativeas SiO₂ (−430 vs. −244 kcal/mole). The boiling point of Si isessentially the same as La at about 3270° C. such that it will notsublime and will not contaminate the Sm metal. However, SiO may form;and it is a gas which possibly may co-sublime with the Sm. It may bepossible to minimize the SiO formed and co-sublimed with the Sm. It isalso possible that small amounts of Si and oxygen (O) will have noappreciable deletious affect on the magnetic properties ofSmCo₅/Sm₂Co₁₇.

Reaction (6) above contrasts to the current process used in the art,which process involves mixing La (or Ce) metal with Sm₂O₃. Because ofthe large vapor pressure differences (boiling point of Sm is 1794° C.and that of La is 3464° C. [Ce—3433° C.]), the Sm metal vaporizesleaving behind La₂O₃ (or CeO_(x′) where 1.5≧x≧2.0). Thereduction/sublimation is carried out at about 800° C.

Another possible advantage of the reduction method of the invention isto reuse the La₂O₃ (or CeO_(x))+Si product in the first step to preparethe LaSi_(0.5) (or CeSi_(0.5)) material (Eq. 1) but less Si needs to beadded since there is free Si in the La₂O₃ (CeO_(x)) product from Eq. 6.If the La₂O₃ (CeO_(x))+Si cannot be reused, the mixture can be dissolvedin an aqueous solution, and the La can be separated from the Si, andconverted to La₂O₃ (CeO_(x)) by conventional chemical procedures.Essentially all of the La₂O₃ (CeO_(x)) is reused, regardless whichrecovery process is used.

This intermediate rare earth-based alloy is useful as a master alloy inmaking a negative electrode material for a metal hydride battery, suchas including but not limited to, R(Ni_(1-x)Co_(x))Si_(0.5) where R isLa, Ce, and MM and x=0 to 0.75. For example, the present inventionprovides in an illustrative embodiment, a method wherein the La-, Ce-,or MM-based intermediate alloy material including Si (as a master alloy)is reacted (alloyed) with a transition metal such as including, but notlimited to, Ni alone or Ni with some portion of the Ni optionallysubstituted by at least one of B, Al, Si, Ti, V, Cr, Mn, Co, Fe, Cu, Zn,and Mo to make an electrode material for a metal hydride battery. Tothis end, the electrode comprises the carbothermically reduced rareearth, Si (or other element selected from the group consisting of Ge,Sn, Pb, As, Sb, Bi, and P) that participated in the carbothermicreduction of the oxide, and the transition metal.

The electrode material incorporating the carbothermically reduced rareearth and one or both of nickel and cobalt can be incorporated into anelectrode using known techniques. The electrode material can be meltedand processed for example by spin casting or other rapid cooling processthat allow for controlled crystallite growth. The cooled material canthen be ground into a powder and mixed with one or more binders. Thematerial can then be pressed, for example on a wire grid or mesh, whichcan then be included in the battery as an electrode, e.g., an anode.

The carbothermic reactions involve the use of a reactant element, suchas one or more elements of Groups IVA (other than carbon), VA of thePeriodic Table of the Elements, oxides thereof, and/or other compoundsthereof. By way of example, in Reactions (1)-(4) silicon is the reactantelement for carbothermic reduction of a rare earth oxide. As a result,the intermediate rare earth-based alloy may include some of the reactantelement, e.g., one or more of silicon, germanium, tin, lead, phosphorus,arsenic, antimony, bismuth, and any products generated from theintermediate rare earth-based alloy may also include the reactantelement. For example, an electrode of a battery that incorporatesmaterial generated from the intermediate rare earth-based alloy mayinclude a reactant element in the carbothermic reaction.

Embodiments of the present invention include an electrode of a batterythat includes a rare earth that has been carbothermically reduced asdescribed above. As noted above, the electrode may include an alloy withthe formula MM(Ni_(1-x)Co_(x))_(4.5)Si_(0.5) where MM is La, Ce and/orother rare earth elements. In this formula, x ranges from about 0 toabout 0.75.

However, the present invention is not limited to any particularstoichiometric composition and may in embodiments include alloys withdifferent compositions. For example, in some embodiments the alloy usedin an electrode, particularly the anode, of a battery includes commonlyat least about 10 wt. %, more commonly at least about 15 wt. %, morecommonly at least about 20 wt. %, and even more commonly at least about30 wt. % of one or both of nickel and cobalt. In some embodiments, thealloy includes commonly up to about 70 wt. %, more commonly up to about45 wt. %, more commonly up to about 40 wt. %, and even more commonly upto about 35 wt. % of one or both of nickel and cobalt. In someembodiments, the alloy includes commonly at least about 5 wt. %, morecommonly at least about 10 wt. %, more commonly at least about 15 wt. %,and even more commonly at least about 20 wt. % of one or more rareearths. In some embodiments, the alloy includes commonly up to about 35wt %, more commonly up to about 40 wt %, more commonly up to about 45 wt% or even more commonly up to about 50 wt % of the rare earth. In someembodiments, the alloy used in an electrode of a battery includesbetween about 20 wt % to about 50 wt % of the rare earth. In someembodiments, the alloy used in an electrode of a battery includescommonly at least about 0.5 wt. %, more commonly at least about 1 wt. %,more commonly at least about 2 wt. %, more commonly at least about 3 wt.%, and even more commonly at least about 4 wt. % of the reactantelement. In some embodiments, the alloy includes commonly up to about 5wt %, more commonly up to about 15 wt %, more commonly up to about 25 wt%, or even more commonly up to about 30 wt % of the reactant element. Inone embodiment, the alloy used in an electrode of a battery includesbetween about 0.5 wt % to about 20 wt % of the reactant element.

The following Examples are offered to illustrate but not limit theinvention:

Example-1 Preparation of La₅Si_(2.5) by a Carbothermic-Silicide Methodfor Nickel-Metal (Lanthanum) Hydride Batteries and Hydrogen StorageAlloys

Preparation of Briquettes

A La₅Si_(2.5) alloy was prepared by carbothermic processing. Forexample, approximately 100 grams of La₂O₃ were obtained from theMaterials Preparation Center at the Ames Laboratory (U.S. Department ofEnergy) (e.g. purity of 95 to 98%) and dried in air for 20 hours at 800°C. A 50.0 gram amount of this oxide was added to 4.5 grams of carbon.The Si was obtained from the Arco Solar Corporation (purity of 99.9% butless pure can be used) and the carbon was obtained from Chevron Chemicaland was of the Shawenigan (acetylene black) type, 100% compressed, −325mesh and contained less than 0.05% ash. This mixture was then blendedfor 2½ hours using a Turbo Blender and mixed with 44 grams of acetonecontaining 3 wt. % polypropylene carbonate (QPAC) to form a pliablemass. Briquettes measured 6 mm thick and ˜13×13 mm were formed, airdried at 100° C. for 2 hours and then stored under vacuum to preventhydrolysis and CO₂ absorption.

Carbothermic Reduction [FRS-43-147RC(La)] to Prepare La₅Si_(2.5)

A 31.5 gram portion of the dried briquettes contained La₂O₃, Si andcarbon (97.5% of stoichiometry) were placed in a 4.1 cm diameter×6.4 cmhigh tantalum (Ta) metal crucible and heated under vacuum using amechanical vacuum pump to 1600° C. and then to 1790° C. for 2 hoursusing both a mechanical vacuum pump and a diffusion vacuum pump (200 to600 μm max pressure). The heating schedule was as follows:

-   -   Heat @ 20° C./min. to 1100° C. and hold for 6 min.    -   Heat @ 10° C./min. to 1400° C. and hold for 6 min.    -   Heat @ 10° C./min. to 1600° C. and hold for 1.5 hrs.    -   Valve in diffusion pump and heat @ 10° C./min. to 1790° C. and        hold for 2 hrs.    -   Cool @ 10° C. to 1600° C.    -   Cool @ 30° C./min. to room temperature.

The prepared alloy was then removed from the Ta crucible and submittedfor chemical analysis and scanning electron microscopic (SEM) analysisto determine the ratio of La to Si. An x-ray analysis was also made.

The alloy was found to contain 0.64 wt. % carbon (C), 0.47 wt. % oxygen(O) and 0.007 wt. % nitrogen (N). The SEM analysis showed it to containan overall La to Si ratio of 5 to 2.5. A mix of La silicides was foundto be present which was confirmed by the x-ray analysis. X-ray analysisshowed that La₃Si₂, La₅Si₃ and La₅Si₄ were present along with smallamounts of La₂O₃ and silicon, no SiC was observed.

The alloy was placed in water for 6 hours, and just a slight reactionwas observed indicating the presence of a small amount of some lanthanumcarbide.

The oxidation of this alloy at room temperature is shown in FIG. 1 alongwith that of a commercial grade La metal. The oxidation resistance isquite low; i.e. a 0.06 weight percent gain after exposure to ambient airfor 1000 hours (more than 40 days) and is comparable to that of normalLa metal.

Preparation of LaNi_(2.25)Co_(2.25)Si_(0.5) (KAA-1-62)

The starting materials were La₅Si_(2.5) (FRS-43-147 from above), as wellas elemental Ni and Co. A 14.7 g button of the stoichiometriccomposition was prepared by arc melting in a partial pressure of argon(Ar). The sample was melted four times at 275 amperes and flipped overbetween each melt. There was minimal weight loss during the meltingprocess. The button was then drop cast at 450 amperes into a chilledcopper mold. After drop casting, a 3 gram portion of the sample waslightly crushed and passed through a 600 μm sieve for hydrogenation.Rietveld refinement of the x-ray diffraction (XRD) powder pattern showedthat the major phase has the LaNi₅ structure.

The LaNi_(2.25)CO_(2.25)Si_(0.5) alloy can be utilized as the negativeelectrode of a Ni-metal-hydride battery, or as a hydrogen storage alloyThe hydrogenation pressure-composition diagram is shown in FIG. 2. Theplateau pressure of 0.5 bar and 3H/formula unit corresponds to a voltageof −0.94 V relative to the mercury oxide/mercury (HgO/Hg) standardelectrode.

The electrical properties of the LaNi_(2.25)CO_(2.25)Si_(0.5) (LaNiCoSi)electrode material were also measured, see FIGS. 3-5. A electrode wasprepared by blending the LaNiCoSi powder with silver powder. TheLaNiCoSi was crushed and screened to a 45 μm particle size, mixed with0.5-1.0 μm silver powder at a 2:1 (LaNiCoSi:Ag) weight ratio, andcompressed at a pressure of ˜1.5 metric tons at room temperature toprovide an irregular shaped plate electrode. The silver powder wasincorporated in the electrode as an electrically conductive binder,although copper powder or other suitable binder can be used to this endas well.

The electrode maintained its structural integrity during all of theelectrical measurements in a test battery cell that comprised a positiveelectrode comprised of Ni oxyhydroxide and a negative electrode (thehydrogen absorbing electrode) comprised of the compressed LaNiCoSipowder with silver powder using a 6 molar KOH solution electrolyte.Testing was conducted at room temperature.

FIG. 3 shows the charging behavior at a current of 20 mA over a 14 hourperiod for the tested electrode.

FIG. 4 shows the discharging characteristics for a discharging currentof 10.6 mA for the tested electrode.

FIG. 5 shows the capacity of the testing cell as a function of thenumber of charging/discharging cycles.

Example-2 Preparation of Ce₅Si_(2.6) by a Carbothermic-Silicide Methodas a Master Alloying Agent

The preparation of the CeO₂+carbon+Si briquettes is the same asdescribed above for La₂O₃, except that commercial grade purity CeO₂ wasused instead of La₂O₃ and the amounts of carbon and Si were adjusted tothe proportions required according to chemical reaction (2).

Carbothermic Reduction [KAA-1-185RC(Ce)] to Prepare Ce₅Si_(2.6) as aMaster Alloying Agent

A set of briquettes weighing 61.1 g containing CeO₂, Si and carbon wereplaced in a Ta crucible of the same dimensions as described above inExample 1 for La, and heated under vacuum using a mechanical vacuum pumpto 1600° C. and then to 1790° C. for 2 hours using both a mechanicalvacuum pump and a diffusion vacuum pump (200 to 600 μm max pressure).The heating schedule was as follows:

-   -   Heat @ 20° C./min. to 1100° C. and hold for 6 min.    -   Heat @ 20° C./min. to 1400° C. and hold for 6 min.    -   Heat @ 10° C./min. to 1600° C. and hold for 1.5 hrs.    -   Valve in diffusion pump and heat @ 10° C./min. to 1790° C. and        hold for 2 hrs.    -   Cool @ 10° C. to 1600° C.    -   Cool @ 30° C./min. to room temperature.

The prepared alloy was then removed from the Ta crucible. An x-rayanalysis showed that this sample contained a mix of Ce silicides—5:3,3:2, 5:4, 1:1 and 1:2 (Ce:Si ratios).

The alloy was found to contain 0.60 wt. % carbon (C), and 0.46 wt. %oxygen (O). The alloy was placed in water for a day and no bubbles ofgas were observed indicating that no Ce carbides were formed.

The master alloy can be utilized as an alloying agent or additive toimprove the metallurgical properties of commercial Mg and Al alloys,cast irons and superalloys, and as a refining agent for steels.

This Ce₅Si_(2.6) alloy is extremely oxidation resistant, much more sothan commercial grade Ce metal, see FIG. 6. Within experimental errorthere was no weight gain when exposed to ambient air for more than 1350hours (56 days).

Example-3 Preparation of MM₄Si₃ by a Carbothermic-Silicide Method as aMaster Alloying Agent

The starting mixed rare earth oxide had a majority of La oxide and Ceoxide collectively with minor amounts of other oxides as shown below:

Oxide Wt. % La₂O₃ 27.1 CeO₂ 55.8 Pr₂O₃ 5.3 Nd₂O₃ 11.7 Sm₂O₃ 0.05 Gd₂O₃0.01 Y₂O₃ 0.01

This mixture is designated as MM₂O₃, where MM=mischmetal.

The preparation of the MM₂O₃+carbon+Si briquettes is the same asdescribed above for La₂O₃, except that commercial grade purity MM₂O₃ wasused instead of La₂O₃ and the amounts of carbon and Si were adjusted tothe proportions required according to chemical reaction (1).

Carbothermic Reduction [KAA-1-211RC(MM)] to Prepare MM₅Si₃ as a MasterAlloying Agent

A set of briquettes weighing 61.3 g containing MM₂O₃, Si and carbon wereplaced in a Ta crucible of the same dimensions as described above inExample 1 for La, and heated under vacuum using a mechanical vacuum pumpto 1600° C. and then to 1790° C. for 2 hours using both a mechanicalvacuum pump and a diffusion vacuum pump (200 to 600 μm max pressure).The heating schedule was as follows:

-   -   Heat @ 20° C./min. to 1100° C. and hold for 6 min.    -   Heat @ 20° C./min. to 1400° C. and hold for 6 min.    -   Heat @ 10° C./min. to 1600° C. and hold for 1.5 hrs.    -   Valve in diffusion pump and heat @ 10° C./min. to 1790° C. and        hold for 2 hrs.    -   Cool @ 10° C. to 1600° C.    -   Cool @ 30° C./min. to room temperature.

The prepared alloy was then removed from the Ta crucible, and was foundto contain 0.55 wt. % carbon (C), and 0.66 wt. % oxygen (O). The alloywas placed in water for a day and no bubbles of gas were observedindicating that no Ce carbides were formed.

The MM₅Si₃ can be alloyed with a Ni master alloy to formMMNi_(4.5)Si_(0.5), which can be utilized as a negative electrode of aNi-metal-hydride battery or as a hydrogen storage alloy, i.e.replacement for La described above under Example 1. The Ni-metal-hydridebattery electrode can include a minor amount of Pr, Nd, Sm, Gd, and/or Ythat originates from the particular MM ore used. The MM₅Si₃ alloy may bealloyed with Fe to manufacture lighter flints, and can be utilized as analloying agent to improve the metallurgical properties of commercial Mgand Al alloys, cast irons and ductile iron, and as a refining agent forsteels.

This MM₅Si₃ alloy is quite oxidation resistant, about the same ascommercial grade MM, see FIG. 7. The weight gain was quite small,˜0.008%, when exposed to ambient air for more than 1350 hours (56 days).

Example-4 Preparation of Y₅Si_(2.6) by a Carbothermic-Silicide Method asa Master Alloy

The preparation of the Y₂O₃+carbon+Si briquettes is the same asdescribed above for La₂O₃, except that commercial grade purity Y₂O₃ wasused instead of La₂O₃ and the amounts of carbon and Si were adjusted tothe proportions required according to chemical reaction (1).

Carbothermic Reduction [KAA-1-215RC(Y)] to Prepare Y₅Si_(2.6) as aMaster Alloy

A set of briquettes weighing 64.4 g containing Y₂O₃, Si and carbon wereplaced in a Ta crucible of the same dimensions as described above inExample 1 for La, and heated under vacuum using a mechanical vacuum pumpto 1650° C. and then to 1800° C. for 1 hour using both a mechanicalvacuum pump and a diffusion vacuum pump (200 to 600 μm max pressure).The heating schedule was as follows:

-   -   Heat @ 20° C./min. to 1100° C. and hold for 6 min.    -   Heat @ 20° C./min. to 1400° C. and hold for 6 min.    -   Heat @ 10° C./min. to 1650° C. and hold for 1.5 hrs.    -   Valve in diffusion pump and heat @ 10° C./min. to 1860° C. and        hold for 6 min.    -   Cool @ 10° C./min. to 1800° C. and hold for 1 hr.    -   Cool @ 10° C. to 1600° C.    -   Cool @ 30° C./min. to room temperature.

The prepared alloy was then removed from the Ta crucible and was foundto contain 1.9 wt. % carbon (C), and 3.2 wt. % oxygen (O). The alloyreacted water and, formed bubbles of gas indicating that some Y carbideswere present, which is consistent with the high carbon content. The highcarbon and oxygen contents along with the existence of Y carbides,suggests that the reaction did not go to completion, primarily becausethe maximum temperature was not sufficient to melt the highest melting Ysilicide, Y₅Si₃ (T_(m)=1875° C.). This was due to the fact that thefurnace being utilized could only reach 1860° C., and the maximumtemperature needed to assure the completion of the reaction is about 50°C. above the highest melting compound in the Y—Si system.

The master alloy can be utilized as an alloying agent to improvecorrosion and oxidation resistance of superalloys and the hightemperature creep resistance to commercial Mg alloys.

The Y₅Si_(2.6) alloy reacts slowly with the ambient air, see FIG. 6,because of the presence of Y carbides in the product. Applicants expectthat the oxidation resistance of Y₅S_(2.6) would be comparable to thatof Y metal, based on experiences with the other three rare earthsilicide master alloys as shown in FIGS. 1, 6 and 7.

Although the present invention has been described above in connectionwith certain illustrative embodiments thereof, those skilled in the artwill appreciate that changes and modifications can be made therein withthe scope of the invention as set forth in the appended claims.

The invention claimed is:
 1. A method of making a rare earth-basedalloy, comprising carbothermically reducing an oxide selected from thegroup consisting of La-containing oxide, a Ce-containing oxide, andMM-containing oxide in the presence of carbon as a reducing agent and asource of a reactant element X at a temperature less than about 1800degrees C. wherein X is selected from the group consisting of Si, Ge,Sn, Pb, As, Sb, Bi, and P to achieve substantial completion of thecarbothermic reduction reaction to form a low carbon alloy thatcomprises a majority of a rare earth element selected from the groupconsisting of La, Ce, and MM, a minor amount of X present in an amountof about 1 weight % to 18 weight % of the alloy, and a low carboncontent in an amount of about 2 weight % or less of the alloy.
 2. Themethod of claim 1 wherein the oxide comprises La₂O₃.
 3. The method ofclaim 2 wherein the alloy comprises LaSi_(0.5).
 4. The method of claim 2wherein the alloy comprises LaX′_(0.5), where X′ is selected from thegroup consisting of Ge, Sn, Pb, As, Sb, Bi, and P.
 5. The method ofclaim 1 wherein the carbothermic reduction is initiated at a temperatureof at least about 1300 degrees C.
 6. The method of claim 1 wherein theoxide comprises CeO₂.
 7. The method of claim 6 wherein the alloycomprises CeSi_(0.5).
 8. The method of claim 6 wherein the alloycomprises CeX′_(0.5), where X′ is selected from the group consisting ofGe, Sn, Pb, As, Sb, Bi, and P.
 9. The method of claim 1 wherein theoxide comprises mischmetal (MM) oxide.
 10. The method of claim 9 whereinthe alloy comprises MMSi_(0.5).
 11. The method of claim 9 wherein thealloy comprises MMX′_(0.5), where X is selected from the groupconsisting of Ge, Sn, Pb, As, Sb, Bi, and P.
 12. The method of claim 1wherein zirconium oxide also is present and reduced.
 13. The method ofclaim 1 wherein the reactant element is Si.
 14. The method of claim 1including providing a substantially stoichiometric amount of carbon as areducing agent relative to oxides present.
 15. The method of claim 1wherein the carbon content of the alloy is in a range of about 0.005weight % to about 2 weight % C.
 16. A method of making a rareearth-based alloy, comprising carbothermically reducing an oxideselected from the group consisting of a La-containing oxide, aCe-containing oxide, and a MM-containing oxide in the presence of carbonparticles as a reducing agent and silicon particles to achievesubstantial completion of the carbothermic reduction reaction at atemperature below 1800 degrees C. to form a low carbon alloy thatcomprises a majority of a rare earth element selected from the groupconsisting of La, Ce, and MM, a minor amount of Si present in a range ofabout 1.0 weight % to about 18 weight % of the alloy, and an amount of Cin a range of about 0.005 weight % to about 2 weight % of the alloy.